audit(#419): declared-only rescore of referee_deployable and plausible_distractor

[maximinl]

Summary

• Offline declared-only audits for MedQA referee_deployable and plausible_distractor (Rescore referee arms from committed caches using declared-only accounting #419), replaying committed caches with zero API calls.
• New scripts write *_declared_audit.json only; historical *_summary.json / .jsonl are untouched.
• Deployable: 39/40 cases fully declared; precision 0.682 → 0.714, clean FP 7 → 6 — headline unchanged.
• Plausible distractor: 31/103 fully declared; adoption gap 0.146 → 0.226, p 0.0059 → 0.039 — still significant at α=0.05, but most cases had at least one undeclared draw (ranking prompts especially).

Notes

• MedMCQA mirrors are ready to run the same scripts once a local manifest is staged; not included here (no MedMCQA raw on this machine).
• MIMIC referee_deployable remains blocked (call cache gitignored).

Test plan

• python -m experiments.referee.declared_only_audit_deployable --manifest <medqa_manifest.csv> (new_api_calls_this_run=0, replay matches committed)
• python -m experiments.medqa.declared_only_audit_plausible_distractor --manifest <medqa_manifest.csv> (same)

Closes #419

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